High Tension
High Tension is a state of powering up using one's own energy within them, immensely boosting his or her physical abilities. It is an ability that lies deep within every Mobian; with the proper training, he or she will be able to utilize their own energy not only to boost their strength, but also manipulate it in ways not unlike Ki. However, the people in Smash's family are somewhat of an exception; they cannot achieve this through regular methods. For them, this potential is locked away until certain requirements are met. The only known one so far is to suffer mortal, life threatening injuries. High Tension then activates for them as a desperation skill, unlocking the potential to use it freely in the future. Ironically, High Tension normally has an emergency switch-off when in such mortal danger, as burning up energy under these conditions is life threatening. While Smash's family has this potential locked away, most other people do not. In turn, however, it's less likely that they'll even know they have such potential.
Even after unlocking the potential, Smash still spent an entire month trying to tap back into it. And when he finally did, it took everything he had to keep the form up for longer than a minute. He also could not control his energy the way others would have been able to, but in turn his power was far greater than the norm.
It would be the same case for his little sister Pearl further down the line, as High Tension in the Corliss bloodline differs from the norm. The imbalance in power and control is natural for them, as is the greater struggle to master the ability.
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High Tension's first stage turns his fur into a sky blue color with a hint of lime green in it. Before mastering High Tension, he constantly struggled to keep the power-up intact for longer than a minute. He could do little more than a few, albiet very powerful attacks before reverting back to normal. He would very often be left winded and in worse cases even vulnerable afterwards. In order to develop this ability further he began doing endurance and stamina training to prolong his High Tension state.
After mastering High Tension, he was able to use this stage without burning up all of his energy, thus allowing him to keep up the form for quite some time before tiring out. While not as powerful as his initial use of it, it became far more effective and efficient. He also learned how to manipulate his energy to create Energy Bursts and Aura Flows to help with mobility and offense.
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The Completed High Tension, (Or Max High Tension) is even more powerful. In this state, he is entirely lime green, with cyan stripes on his arms and legs. Max High Tension is an ability unique to the Corliss bloodline, and is proof of all the training and harships Smash has been through. His physical abilities increase dramatically, and skills such as his Energy Bursts become even more potent. He burns up more energy in this form, like he used to in his previous stage. He can maintain this form longer than before, but it still doesn't last too long if he keeps it up. This is why he tries not to use it unless needed, or if he's going for a finishing move. If he lets this form go on for too long, he'll subconsciously revert back before he puts himself in danger from burning up too much energy. If this happens, he'll suffer from extreme exhaustion.
It is possible for him to bypass his subconscious reversion, however it is extremely dangerous. Prolonged usage will eventually drain all of his life energy, and will most definitely be fatal.
Gundam 00 OST 2 Track 20 - Fight
Smash's "High Tension" theme
Super High Tension is essentially what happens when Super Smash taps into his High Tension state, or when Smash performs a Super Transformation while in High Tension. It is all around a very powerful form that could potentially increase his strength ten fold. However, it was very quickly dismissed as impractical, as the Chaos Energy empowering his super form conflicts with his focus on tapping into his own life energy for High Tension. In order to maintain control of both powers, he would have to be so focused on holding them together that he wouldn't be able to put it to any good use in a real battle. One mistake and he might revert back to normal entirely. It would only be useful in dealing a finishing blow on an immobilized target.
Super Smash
Super Smash and Power Pearl
With the power of the Seven Chaos Emeralds, or something of equal power, Smash is able to transform into Super Smash. His power increases greatly, his jumping capabilities are incredible, and he floats gently downward like a feather. He can also hover just above the ground, or glide through the air, but can't actually fly. His fur turns gold in this form, and his eyes become cyan. Despite that, though, they appear as green, due to the golden aura surrounding him. Through experimenting with this form, he was able to achieve High Tension for the first time.
After his training with Angelus, he was able to perform the same unique abilities from High Tension in his regular Super form. It's far more effective now, since the energy is not his own. He is free to burn it up as recklessly as he wants with little consequences other than depleting the form. He faces very little withdrawel from reverting back to normal in comparison to High Tension.
Ultra Smash
Voted Best Superform for Spring 2013! 
Ultra Smash, the strongest of his super forms.
Smash has one more form that few people have ever seen. With the power of the Seven Super Emeralds, the powered up versions of the Chaos Emeralds, or something equal to that amount of power, Smash is able to advance to his most powerful form: Ultra Smash. Like Super Smash, His fur is golden, but he also has green stripes on his arms and legs, and a green aura surrounds him. What's more, his hair grows out longer and wilder, becoming spiky and discheveled, and two large spikes grow out his knuckles on each hand. His power increases dramatically compared to Super Smash. This is the only form that grants him true flight. He isn't quite as fast as he normally would be, but he makes up for it with raw power and great durability. His energy based attacks aren't as effective as his Light form's, but they are still quite powerful. Though his powerful finisher, Divine Fist, is an exception. Its power is far beyond anything he had ever, and ever will dish out. An attack this powerful would destroy his own body, effectively becoming a sacrifical attack.
There is a very short (and very old) video clip of how he transforms into Ultra Smash. He first uses his own strength to go into his High Tension state, then he uses the power of the emeralds to go super, and then he slowly grows longer hair and spikes before completing the transformation with an incredible blast of energy. And this is just the shorter version. The transformation is actually longer than that, but due to the RPGMaker battle animation limitations, this was as long as it could be made. Though a longer version is in the process.
Scrapped Forms
Mega Smash
This form only appeared once in one of the many Lost Episodes. Mega Smash was a powerful transformation, second only to Ultra Smash. His color changed to green, and he had incredible flight power. But what was really striking about this form, was that he grew nearly twice his original size. The form was never actually used in battle, as the enemy in question fled upon seeing it. It was more or less used as a threat.
Exactly how he transformed into this is unknown. The Super Emeralds may have been involved somehow, but that would have changed him into Ultra Smash, instead.
Chaos Smash
A form that barely anyone remembers. He had been gifted with a "Chaos" form from someone else in an RP on an SRB2 netgame. Once again, he was green; A sort of pale, dull shade of green in contrast to Mega Smash's bright green color. In this form he was able to use just about as many chaos powers as he could think of. However, he wasn't very knowledgeable with them, so it didn't really change much.
The form was dropped sometime later. He didn't really have a need for it, anyway.
Ultra Light Smash
Now known as Light Smash. Basically, Ultra Light Smash was the same as it is today, except more powerful than Ultra Smash. (In fact, Ultra Smash wasn't even as strong as it is now.) It was supposed to be an awesome, past the limit of final super transformations. It ended up being pretty godly, so I toned it down. But even after that, the name still made it sound more powerful than it was. Finally, it got a make over and turned into regular Light Smash.
Light Smash
A drawing of Light Smash in his older design. (Credit to Super Lightning)
Light Smash was, for a time, the most powerful level of High Tension. The requirements were essentially that of a Super Saiyan's first transformation, and I didn't really want that. It also debuted before High Tension, heavily implying that Smash had already held great power within him from the start. While that was an interesting concept, I didn't want that. Light Smash didn't show up enough to be really relevent anyway.
One could argue the same thing about Ultra Smash as well, considering that the only RP's he showed up in became non-canon. I'm still actively trying to find a use for him.
Dark Ultra Smash
Probably the most commonly used of the scrapped forms. This was Smash's dark form, which unlike the name implied, was not as powerful as Ultra Smash or the opposite of Ultra Light Smash. Though in terms of activation, it kind of was the opposite. Dark Ultra Smash's fur was dark blue, and his eyes were blood red. The color of whatever clothing he wore also darkened. This form was pretty much powered by his hatred, somewhat like Darkspine Sonic. His attitude changed completely, as he turned cold and ruthless.
Modern D.U Smash, as seen in SpTH
Eventually, he ended up being able to control this form quite like his current High Tension state; in that he could go in and out of it at will. It did sap a lot of his energy from him, too. During it's final stages, Dark Ultra Smash was actually one of the most tolerable out of all the scrapped forms before it joined the others. That's why it's possible that it may come back again in some way, with a whole story arc surrounding it. It's still just a possibility, however.
Dark Ultra Smash reappears in Splice The Hedgehog as an optional boss battle in a bonus dungeon.
Dark Shadow Smash
This was the dark equivalent of Ultra Light Smash. Dark Shadow Smash was completely black with extremely long spikes on his knuckles. Even his shoes grew spikes. He was always surrounded by a pitch black aura, and his power was immense. Unlike Dark Ultra Smash, where he had control over his form, Dark Shadow Smash was completely berserk and attacked anything that moved like a wild animal. He was considered to be "unstoppable" because no one could even approach him in this form.
This form was scrapped before D.U Smash, seeing as it was too powerful and the requirements were virtually unobtainable unless some grave, massively story-changing events happened.
- Arc Aungel was the name of a character belonging to E-113:Xi. The character was eventually changed into Micheal, who went by the name "Angelus" in the Cosmic Championship, and his sole role was to train Smash during the tournament. He himself makes little to no other appearances throughout the rest of the series, but his past and backstory is revealed later on.